1. Field of the Invention
This invention relates to apparatus of the type having separate members that are movable guidingly relative to each other around a rotational axis and, more particularly, to such an apparatus in which a coil spring interconnects between the separate members so as to transmit rotational forces therebetween.
2. Background Art
In automobile engines, it is well known to provide an alternator that is driven by the engine crankshaft to generate power. However, due to the nature of internal combustion engines, the crankshaft rotation will fluctuate, as a result of which there are recurring and momentary increases and decreases in rotational speed.
The power generating shaft of the alternator has a large moment of inertia. Typically, an endless power transmission belt transmits power from the engine crankshaft to a pulley on the alternator shaft. With this type of system, slippage commonly occurs between the pulley and belt each time the speed of the belt changes by reason of fluctuations in the rotational speed of the engine crankshaft. This condition can cause noise as the belt slips with respect to the pulley.
A further problem resulting from fluctuations in the crankshaft speed is that a corresponding fluctuation in the alternator shaft may adversely affect power generation capability, thereby resulting in a reduction in power generation efficiency.
It is known to absorb these system fluctuations by elastically connecting the alternator shaft to the associated pulley. Through this arrangement, fluctuations may be absorbed without slippage occurring between the belt and pulley on the alternator shaft. It is known to incorporate an elastic member and/or a viscous fluid between the pulley and a shaft relative to which the pulley rotates on the alternator.
One example of such a hybrid system is disclosed in JP 08-240246 A. In this system, a rubber elastic member is interposed between the pulley and a separate member relative to which the pulley is rotated. A viscous fluid is also utilized, the viscosity of which increases in response to the generation of a shear force resulting from rotational speed fluctuations.
With this construction, even when a torque is applied to the pulley, capable of generating a shearing stress in excess of the elastic limit of the elastic member, a relative angular displacement between the pulley and member is suppressed by reason of an increase in viscosity of the viscous fluid. This avoids damage to the elastic member due to yielding, or breakage.
However, the elastic deformation permissible within the limit of the elasticity of the rubber member may not be such as to permit the desired degree of relative angular displacement between the pulley and the member relative to which it rotates.
Another drawback with this type of system is that the fluctuation in torque resulting from crankshaft rotational speed fluctuations is not easily transmitted to the alternator shaft. At the same time, the belt is subject to resonance due to fluctuation in tension, so that additional noise may be generated and/or the durability of the belt may be adversely affected.
It is also known to use only an elastic member between the pulley and member relative to which the pulley rotates. The elastic member is commonly in the form of a coil spring, the ends of which are fitted in, and fixed with respect to, arcuate grooves provided in each of the pulley and cooperating member. The ends of the coil spring are curved and locked to their respective members. In the regions of the grooves where the coil spring is not fixed, there are normally fixed gaps between the spring and the surfaces bounding the grooves.
With this latter construction, it is possible to allow a relatively large angular displacement between the pulley and cooperating member, whereby it may be possible to reduce fluctuation in the tension of the belt wrapped around the pulley. As a result of this, belt squealing may be suppressed. This system may also account for improved durability of the belt.
U.S. Pat. No. 5,139,463 discloses a pulley used on an alternator cooperating with a serpentine drive mechanism on an automobile engine. The system has a hub that rotates together with an armature assembly and an AC generator pulley mounted on the hub. Between the hub and the AC generator pulley, a coil spring is provided having end portions that are fixed to the hub and pulley. Rotational movement of the AC generator pulley through a serpentine belt is transmitted to the hub. It is also possible to effect relative elastic rotational movement in the opposite direction with respect to the AC generator pulley. The end portions of the coil spring are bent radially outwardly and are accommodated in complementary notches in the hub and AC generator pulley.
By using this type of system, a number of the aforementioned problems associated with the system in JP 08 240246 A are avoided. That is, belt resonance and resulting noise may be avoided. Further, the belt durability problems associated with this prior system are potentially avoided.
However, by reason of increasing the relative angular displacement of the pulley and power generation shaft of the alternator, the coil spring may be prone to failure, for the reasons stated below. At the regions where the coil spring ends are fitted and fixed to their respective members, adjacent to the regions where the above-noted gaps are provided, there are sharp transition corners. That is, the ends of the coil spring are each bent at approximately a right angle at this location. Each time the coil spring undergoes elastic deformation, there is a stress concentration in the vicinity of the corners. As a result, due to the localized stress concentration resulting each time the crankshaft rotational speed fluctuates, there is a danger that the corners on the coil spring ends may fatigue to the point of fracture. As a result, the coil spring may break in a relatively short period of time, as a consequence of which the overall structure may become inoperative.